1. Field of the Invention
The present invention relates generally to a power distribution control system for four wheel drive vehicle. More specifically, the invention relates to a technology for preventing the power distribution control system from causing hunting in adjusting proportion of power distribution for front and rear wheels.
2. Background of the Invention
Japanese Patent First (unexamined) Publication (Tokkai) Showe 61-157437 discloses a power distribution control system for a four wheel drive vehicle. The power distribution control system disclosed in this prior publication, proposes power distribution control for prevention of wheel spin, unbalanced wheel lock at left and right sides of the vehicle during abrupt braking, wheel slippage on low friction road and so forth. In the disclosed system, power distribution is adjusted on the basis of wheel speed difference between primary and auxiliary driving wheels, i.e. front and rear wheels. That is, when the wheel speed difference becomes greater, the proportion of power distribution for the auxiliary driving wheels is increased toward an even ratio to the primary driving wheels, and a smaller wheel slip difference results in smaller proportion of driving power to be distributed to the auxiliary driving wheels.
In such prior proposed power distribution control systems, power distribution is controlled in a feedback system with taking the wheel speed difference between the primary and auxiliary driving wheels as feedback parameter. A problem has been encountered at the occurrence of four wheel slippage. In such occasion, since wheel speed difference fluctuates depending upon the power distribution ratio for the auxiliary wheels because of substantially equal magnitude of wheel slippage at both of primary and auxiliary driving wheels. Fluctuation of wheel speed difference results in variation of the power distribution ratio for the primary and auxiliary driving wheels. Hunting is thus caused. Hunting results in self-induced noise or jerking of the vehicle.
For example, when the vehicle has a power train layout taking a rear wheel as a primary driving wheel, and abrupt acceleration is demanded on the low friction road, wheel slippage at rear wheels increases abruptly to increase the wheel speed difference. According to an increasing of wheel speed difference, a change of the power distribution ratio for increasing power distribution for the front wheel as the auxiliary driving wheel is commanded. Since the front wheel has substantially small inertia, wheel speed at the front wheel is rapidly increased. Therefore, such change of power distribution is taken place abruptly for rapidly decreasing wheel slippage difference. This results in abrupt change of power distribution ratio for decreasing distribution ratio for the auxiliary driving wheel. An abrupt change of power distribution causes recovery of traction at the front wheel thereby causing an abrupt drop of the wheel speed at the front wheel. Therefore, again, the wheel speed difference at front and rear wheels is abruptly increased.
The tendency of causing hunting as set forth above may be increased when control gain for wheel speed difference dependent power distribution control is increased. On the other hand, if the control gain is reduced, response ability in wheel speed difference dependent power distribution control can be lowered for degrading vehicular acceleration characteristics and vehicular driving stability.